1. Field of the Invention
The present invention relates to a plastic sheet with a ruled line for bending. More particularly the present invention relates to the structure of a ruled line which forms a bent portion when a plastic sheet is bent.
2. Discussion of Background
In bending a plastic sheet to form a container for packaging, it is a usual way to form a ruled line for bending in the sheet and to bend the plastic sheet along the ruled line. Usually, a plastic sheet with a ruled line in its one surface has been used.
In the conventional plastic sheet with a ruled line for bending in its one surface, there was a problem that it was difficult to bend the sheet because corner portions in the width direction of the ruled line come to contact with the surface of the plastic sheet when it was bent. In order to solve the problem, it is considered that the width of the ruled line is broadened. However, the formation of a wide and deep ruled line in the plastic sheet requires that a press mold for forming a ruled line has to be pressed under a high pressure to the plastic sheet, which may break the sheet depending on the physical properties of the sheet. Further, when a wide ruled line is formed, the plastic sheet is stretched in the lateral direction. Accordingly, when a small-sized box is manufactured from such plastic sheet, there may result a wrinkle and waviness at a portion where ruled lines are crossed.
In order to solve the above-mentioned problem, there is a proposal of a plastic sheet with a ruled line as shown in FIG. 6. Namely, the plastic sheet has a groove as a ruled line for bending in which shallow recess portions 2 and deep recess portions 3 are alternately formed in the longitudinal direction of the bottom portion of the groove by providing convex and recess portions or discontinuous holes (Japanese Examined Utility Model Publication No. 9345/1992).
Although the plastic sheet having the above-mentioned construction can provide easy bending characteristic, there was a problem as follows. When the plastic sheet is bent along a ruled line 1, both end portions 2a, 2a of the bottom of the shallow recess portions 2 are exposed with sharp ridges as shown in a perspective view in FIG. 7. Accordingly, a touch feel to these portions is no good and if cloth or the like is put in the container fabricated with a plastic sheet, there is a danger of damaging the cloth because a string of the cloth cross is caught by a sharp ridge in the ruled line.